A lithium secondary battery and its application

Abstract

This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium secondary battery and its application. The battery pack includes alternating positive and negative electrodes, with the outermost layer of the battery pack being entirely positive electrodes, and a separator containing an electrolyte disposed between the positive and negative electrodes. The positive electrode comprises lithium manganese iron phosphate, and the electrolyte comprises an electrolyte, an organic solvent, and additives. The additives contain tris(trimethylsilane) phosphate, with a mass content of 0.5-3 wt%. This invention significantly improves the cycle stability of the battery and substantially enhances the cycle life of lithium secondary batteries using lithium manganese iron phosphate positive electrode materials.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium secondary battery and its application. Background Technology

[0002] In the field of lithium-ion batteries, lithium manganese iron phosphate (LMFP) is considered a promising cathode material due to its high voltage platform, good thermal stability, and cost-effectiveness. However, lithium-ion batteries with LMFP cathodes often face problems of insufficient cycle life and cycle stability during cycling, which limits their widespread use in long-life applications.

[0003] While existing technologies can achieve the basic functions of lithium-ion batteries using lithium manganese iron phosphate (LFP) cathode materials, several problems and shortcomings remain. First, traditional electrolytes lack effective additives, failing to effectively suppress side reactions during cycling, leading to rapid performance degradation. Second, existing battery structures cannot fully utilize the potential of the cathode material, limiting cycle stability. Furthermore, current technologies perform poorly under high temperature and high voltage conditions, failing to meet the growing demand for high-performance batteries. Therefore, developing a battery structure to improve the cycle life and cycle stability of LFP-based lithium-ion batteries has significant practical importance and application value.

[0004] It should be noted that this part of the present invention only provides background technology related to the present invention, and does not necessarily constitute prior art or known technology. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing lithium secondary batteries using lithium manganese iron phosphate cathode materials, such as low cycle life and poor cycle stability, and to provide a lithium secondary battery and its application. This invention significantly improves the cycle stability of the battery and greatly enhances the cycle life of lithium secondary batteries using lithium manganese iron phosphate cathode materials, while still exhibiting excellent cycle life and cycle stability at high temperatures.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a lithium secondary battery, comprising a battery pack, the battery pack including alternating positive and negative electrodes, wherein the outermost layer of the battery pack is all positive electrodes, and a separator containing an electrolyte disposed between the positive and negative electrodes; wherein the positive electrode includes lithium manganese iron phosphate positive electrode material, the electrolyte includes an electrolyte, an organic solvent and an additive, wherein the additive contains tris(trimethylsilane) phosphate, and the mass content of the tris(trimethylsilane) phosphate is 0.5-3 wt%.

[0007] In some preferred embodiments of the present invention, the additive also contains vinylene carbonate.

[0008] In some preferred embodiments of the present invention, the mass ratio of vinylene carbonate to tris(trimethylsilane) phosphate is (0.5-2):1.

[0009] In some preferred embodiments of the present invention, the mass content of vinylene carbonate in the electrolyte is 0.25-6 wt%.

[0010] In some preferred embodiments of the present invention, the electrolyte comprises lithium hexafluorophosphate, wherein the mass ratio of lithium hexafluorophosphate to tris(trimethylsilane) phosphate is (5-35):1.

[0011] In some preferred embodiments of the present invention, the mass of lithium hexafluorophosphate is 10wt%-17wt% of the total mass of the electrolyte.

[0012] In some preferred embodiments of the present invention, the organic solvent comprises cyclic carbonates and / or chain carbonates; wherein,

[0013] Cyclic carbonates include at least one of ethylene carbonate, propylene carbonate, and butene carbonate, while chain carbonates include at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0014] And / or, when the organic solvent includes cyclic carbonates and chain carbonates, the mass ratio of cyclic carbonates to chain carbonates is (0.5-5):1.

[0015] In some preferred embodiments of the present invention, the lithium secondary battery further includes at least one of the following structures:

[0016] Structure 1: Both the positive and negative electrodes are sheet-like electrodes, and / or the battery pack is a stacked structure formed by the positive and negative electrodes and the separator;

[0017] Structure 2: The negative electrode includes a negative electrode active material, wherein the negative electrode active material includes at least one of carbon, silicon carbon, and silicon negative electrode.

[0018] Structure 3: The lithium secondary battery also includes a battery casing, and the battery pack is installed inside the battery casing.

[0019] Preferably, the lithium manganese iron phosphate cathode material comprises a lithium manganese iron phosphate matrix and a carbon coating layer covering the surface of the lithium manganese iron phosphate matrix; the chemical formula of the lithium manganese iron phosphate matrix is ​​Li. a Fe x Mn y M j PO qWherein, M includes at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, and La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, and 4≤q≤5; the mass of the carbon coating layer accounts for 0.5%-3% of the total mass of the lithium manganese iron phosphate cathode material.

[0020] In a second aspect, the present invention provides an electrical device comprising the lithium secondary battery described in the first aspect.

[0021] Beneficial effects:

[0022] The battery casing of a lithium secondary battery is usually suspended in the air, but it forms capacitive coupling with the outermost electrode plate through media such as thermally conductive adhesive and insulating film, or there may be a small leakage current, thus forming a "bias voltage". When the outermost electrode is the negative electrode, the battery casing potential is close to a low potential (0.1-0.5V vs. Li / Li). + This can cause instability and reductive decomposition of solvent molecules and lithium salt anions in the electrolyte, consuming active lithium and electrolyte, generating gas, and damaging the stability of the SEI film, leading to problems such as decreased battery capacity and swelling. When the outermost electrode is the positive electrode, the battery casing potential is close to a high potential (3.8-4.2V vs. Li / Li+). Although there is a risk of electrolyte oxidation and decomposition, this is mitigated by the inner layer of the aluminum-plastic film or the passivation layer of the metal casing, and the positive electrode CEI film, thus minimizing the harm. The electrochemical stability window of conventional carbonate electrolytes is 1.0-4.5V (vs. Li / Li+). + The negative electrode side shell potential far exceeds the lower limit of the reduction potential, resulting in violent decomposition. The positive electrode side shell potential does not exceed the upper limit of the oxidation potential by much, and the oxidation decomposition rate is relatively slow.

[0023] Based on this, the present invention, through the above-mentioned technical solution, especially for lithium secondary batteries using lithium manganese iron phosphate cathode material, sets up a stacked structure in which the outermost layer of the battery pack is all positive electrode with alternating positive and negative electrodes, and incorporates tris(trimethylsilane) phosphate (TMSP) as an additive. This structure can fully utilize the potential of lithium manganese iron phosphate cathode material. The addition of TMSP to the electrolyte can preferentially oxidize and form a stable CEI film on the positive electrode, effectively suppressing side reactions during battery cycling. Furthermore, it exerts a synergistic amplification effect in the battery pack structure where the outermost layer of the battery pack is all positive electrode, thereby significantly improving the cycle stability of the battery and significantly enhancing the cycle life of lithium secondary batteries using lithium manganese iron phosphate cathode material. Moreover, the negative electrode material of the present invention has a wide range of choices, including carbon, silicon-carbon, and silicon negative electrodes, enabling the battery to adapt to different application scenarios and requirements.

[0024] The addition of TMSP to the electrolyte plays a synergistic amplification role in battery pack structures where the outermost layer is entirely positive. One hypothetical principle is that even though the outermost layer of the battery pack is positive, preventing electrolyte reduction reactions at the casing, slow oxidation and decomposition still occur at the interface between the casing and the electrolyte under long-term high voltage. Furthermore, degradation issues such as transition metal dissolution can occur during positive electrode cycling, and the resulting harmful substances can damage the SEI film of the negative electrode and catalyze electrolyte decomposition. Additionally, the HF produced by lithium salt decomposition... It will corrode various battery components; while the electrolyte additive contains tris(trimethylsilane) phosphate, which: 1. Strengthens the "shell-electrolyte" interface: TMSP preferentially migrates to this high-potential "shell-electrolyte" interface and undergoes oxidative decomposition there, forming a thin and dense protective film. This film effectively blocks the direct contact between the subsequent electrolyte solvent and the high-potential shell surface, fundamentally inhibiting the continuous oxidation side reaction at this location; 2. Stabilizes the cathode material and reduces "contamination sources": The P=O and Si-C bonds in the TMSP molecular structure enable it to undergo oxidative decomposition before conventional electrolyte solvents (such as EC, DEC), forming a stable, phosphorus- and silicon-rich interface protective film (CEI film) on the cathode material surface, which can reduce the dissolution of transition metals and effectively inhibit the side reactions of lithium manganese iron phosphate cathode material during cycling; 3. Removes harmful substances: It can also capture and remove HF through the phosphorus and oxygen atoms in the TMSP molecule. The H2O component protects the overall system; combined with the structural optimization that "the outermost layer of the battery pack is all positive electrode," the synergistic effect of physical structure and interfacial chemistry achieves battery system stability and improves cycle performance. Furthermore, the solution of this invention maintains excellent battery performance even under high temperature and high voltage conditions, especially in cycle stability and cycle life.

[0025] In a preferred embodiment of the present invention, by adding TMSP (tris(trimethylsilane)phosphate) and VC (ethylene carbonate) to the electrolyte additive, the synergistic effect of TMSP and VC promotes the formation of a film on the positive electrode, further effectively suppressing side reactions of the lithium manganese iron phosphate positive electrode material during cycling, while simultaneously forming a film on the negative electrode, thereby significantly improving the cycle life of the battery. Furthermore, controlling the preferred specific ratio of TMSP and VC in the additive (more preferably combined with a reasonable combination of organic solvents) facilitates the formation of good CEI and SEI films, helping to reduce performance degradation of the battery during cycling and maintaining the long-term stability of the battery. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a capacity retention curve of Embodiment 2 of the present invention at room temperature.

[0028] Figure 2 This is a capacity retention curve of Embodiment 2 of the present invention at high temperature.

[0029] Figure 3 This is a capacity retention curve of Comparative Example 1 of the present invention at room temperature.

[0030] Figure 4 This is a capacity retention curve of Comparative Example 1 of the present invention at high temperature.

[0031] Figure 5 This is a schematic diagram of a stacked lithium secondary battery structure in a specific embodiment of the present invention.

[0032] Figure 6 This is a schematic diagram of a wound-type lithium secondary battery in one specific embodiment of the present invention.

[0033] Explanation of Figure Numbers

[0034] 1. Positive electrode, 2. Negative electrode, 3. Separator, 4. Battery casing. Detailed Implementation

[0035] In this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0036] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0037] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges. For numerical ranges, the endpoint values ​​of the ranges, the endpoint values ​​of the ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "optional" mean that they may or may not be included (or may or may not be present).

[0038] In a first aspect, the present invention provides a lithium secondary battery, comprising a battery pack, the battery pack including alternating positive and negative electrodes, wherein the outermost layer of the battery pack is all positive electrodes, and a separator containing an electrolyte disposed between the positive and negative electrodes; wherein the positive electrode includes lithium manganese iron phosphate positive electrode material, and the electrolyte includes an electrolyte, an organic solvent and an additive, wherein the additive contains tris(trimethylsilane) phosphate.

[0039] This invention features a unique design where the outermost layer of the battery pack is entirely positive, with the battery casing potential approaching a high potential (3.8-4.2V vs. Li / Li+). Although there is a risk of electrolyte oxidation and decomposition, this is mitigated by the inner aluminum-plastic film or the passivation layer of the metal casing, and the positive electrode CEI film. Therefore, compared to conventional designs where at least one outermost electrode is negative, the risk is significantly reduced. Furthermore, the electrochemical stability window of conventional carbonate electrolytes is 1.0-4.5V (vs. Li / Li+). + The negative electrode side shell potential far exceeds the lower limit of the reduction potential, leading to violent decomposition. The positive electrode side shell potential does not exceed the upper limit of the oxidation potential by much, resulting in a slower oxidation decomposition rate. This invention also incorporates tris(trimethylsilane)phosphate (TMSP) into the electrolyte. The P=O and Si-C bonds in the TMSP molecular structure cause it to undergo oxidative decomposition before reaching conventional electrolyte solvents (such as EC and DEC), forming a stable, phosphorus- and silicon-rich interfacial protective film (CEI film) on the surface of the positive electrode material. When this characteristic is applied to a battery pack structure where the outermost layer is all positive electrode, a synergistic amplification effect is generated, effectively suppressing side reactions during battery cycling, significantly improving battery cycle stability, and significantly enhancing the cycle life of lithium secondary batteries using lithium manganese iron phosphate positive electrode materials. In summary, the solution of this invention achieves a synergistic effect of physical structure and interfacial chemistry, enabling the "outermost layer of the battery pack being all positive electrode combined with the addition of TMSP to the electrolyte" approach to deliver remarkable results beyond any single measure, ensuring the ultimate stability of the weakest link.

[0040] Preferably, the mass content of tris(trimethylsilane)phosphate in the additive is 0.5-3 wt%, specifically 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.4 wt%, 1.5 wt%, 1.7 wt%, 1.9 wt%, 2 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt%, 2.7 wt%, 2.8 wt%, 2.9 wt%, or 3 wt%, or any range between any two values.

[0041] In some preferred embodiments of the present invention, the additive further contains vinylene carbonate. Vinylene carbonate can synergistically promote the formation of a stable and dense CEI film on the positive electrode with TMSP, further effectively suppressing the side reactions of the lithium manganese iron phosphate positive electrode material during cycling, while also forming an SEI film on the negative electrode, thereby significantly improving the cycle life of the battery.

[0042] In some preferred embodiments of the present invention, the mass ratio of vinylene carbonate to tris(trimethylsilane) phosphate is (0.5-2):1, specifically 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, or 2:1, or any range between any two values. The present invention uses this preferred and suitable ratio of vinylene carbonate to tris(trimethylsilane) phosphate, which facilitates the formation of a more stable and dense CEI film and SEI film, helping to further reduce battery performance degradation during cycling and maintain long-term battery stability.

[0043] In some preferred embodiments of the present invention, the mass content of vinylene carbonate in the electrolyte is 0.25-6 wt%, specifically, it can be 0.25 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.4 wt%, 1.5 wt%, 1.7 wt%, 1.9 wt%, 2 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt%, 2.7 wt%, 2.8 wt%, 2.9 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 5.9 wt%, or 6 wt%, or any range between any two points.

[0044] The electrolyte contains an electrolyte. The electrolyte can be a conventional electrolyte salt from the prior art. Preferably, the electrolyte comprises lithium hexafluorophosphate. Lithium hexafluorophosphate exhibits excellent overall performance in conventional carbonate solvent systems. It has high solubility and ionic conductivity in commonly used organic carbonate solvents (such as EC, DMC, DEC, and EMC), and can form a stable interface film (SEI / CEI) with graphite anode and high-voltage cathode materials. These characteristics are crucial for ensuring the high-rate charge / discharge performance, long cycle life, and safety of the battery.

[0045] Preferably, the mass ratio of lithium hexafluorophosphate to tris(trimethylsilane)phosphate is (5-35):1, specifically, it can be 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1, 21:1, 22:1, 23:1, 24:1, 25:1, 26:1, 27:1, 28:1, 29:1, 30:1, 31:1, 32:1, 33:1, 34:1, or 35:1, as well as any range between two points. By adopting this preferred embodiment of the present invention, it is more conducive to fully leveraging the synergistic amplification effect of the specific structure of lithium manganese iron phosphate cathode material, the addition of TMSP to the electrolyte, and the fact that "the outermost layer of the battery pack is all positive electrode", thereby significantly improving the cycle stability of the battery and further enhancing the cycle life of lithium secondary batteries using lithium manganese iron phosphate cathode material.

[0046] In some preferred embodiments of the present invention, the mass of lithium hexafluorophosphate is 10wt%-17wt% of the total mass of the electrolyte.

[0047] In some preferred embodiments of the present invention, the organic solvent may include cyclic carbonates and / or chain carbonates. More preferably, the cyclic carbonates include at least one selected from ethylene carbonate, propylene carbonate, and butenyl carbonate. More preferably, the chain carbonates include at least one selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0048] In some preferred embodiments of the present invention, the organic solvent includes cyclic carbonates and chain carbonates. Compared to a single cyclic carbonate or a single chain carbonate, the combination of cyclic carbonates and chain carbonates in the organic solvent can address the balance between ion dissociation and ion transport, ultimately achieving a balance between high conductivity, low viscosity, and good wettability in the electrolyte with added TMSP under high pressure. Combined with a specific positive electrode as the outermost layer of the lithium secondary battery structure, this further improves the battery's cycle stability and low-temperature discharge performance, avoiding the shortcomings of a single solvent (cyclic carbonates have high viscosity, chain carbonates have low dielectric strength).

[0049] More preferably, the mass ratio of cyclic carbonate to chain carbonate is (0.5-5):1, specifically, it can be 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 4.9:1, or 5:1, or any range between any two values. This preferred embodiment of the invention further improves battery cycle stability and low-temperature discharge performance.

[0050] In some preferred embodiments of the present invention, both the positive and negative electrodes are sheet-like electrodes. This preferred embodiment facilitates the formation of a sheet-like stacked structure.

[0051] Preferably, in this invention, the battery pack is a stacked structure formed by a positive electrode, a negative electrode, and a separator. This structure better leverages the synergistic amplification effect of the lithium manganese iron phosphate positive electrode material, the addition of TMSP to the electrolyte, and the fact that "the outermost layer of the battery pack is all positive electrodes," thereby significantly improving the cycle stability of the battery. The stacked structure of this invention can be specifically as follows: Figure 5 The stacked battery shown can also be specifically as follows: Figure 6 The wound battery shown, or cylindrical wound battery (not shown), can be used in various forms of batteries such as pouch batteries and button batteries.

[0052] In some preferred embodiments of the present invention, the negative electrode comprises a negative electrode active material. More preferably, the negative electrode active material comprises at least one of carbon (such as graphite), silicon-carbon, and silicon negative electrode. The present invention offers a wide range of choices for the negative electrode active material, enabling the battery to adapt to different application scenarios and requirements.

[0053] In some embodiments, the negative electrode further includes a negative electrode current collector, which may include: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof. The negative electrode active material is loaded onto the negative electrode current collector. The positive electrode material of the present invention is loaded onto the positive electrode current collector. The loading density, i.e., the coating surface density, of the positive and negative electrode materials can be within the range of the prior art and can be used in the present invention, and will not be elaborated further here.

[0054] In this invention, a separator is provided between the positive and negative electrodes to prevent short circuits. The material and shape of the separator used in the embodiments of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be selected.

[0055] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic materials. The inorganic layer includes inorganic particles and a binder. The inorganic particles include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The polymer layer contains a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0056] Preferably, the lithium manganese iron phosphate cathode material comprises a lithium manganese iron phosphate matrix and a carbon coating layer covering the surface of the lithium manganese iron phosphate matrix; the chemical formula of the lithium manganese iron phosphate matrix is ​​Li. a Fe x Mn y M j PO q Wherein, M includes at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, and La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, and 4≤q≤5; the mass of the carbon coating layer accounts for 0.5%-3% of the total mass of the lithium manganese iron phosphate cathode material.

[0057] In this invention, in the chemical formula of the lithium manganese iron phosphate matrix, 0.8≤a≤1.2, for example, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, and any value within the range of any two values ​​between them.

[0058] In this invention, the chemical formula of the lithium manganese iron phosphate matrix contains 0.1 ≤ x ≤ 0.9, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and any value within a range of any two values ​​between them.

[0059] In this invention, the chemical formula of the lithium manganese iron phosphate matrix contains 0.1 ≤ y ≤ 0.9, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and any value within the range of any two values ​​between them.

[0060] In this invention, the chemical formula of the lithium manganese iron phosphate matrix contains 0 ≤ j ≤ 0.1, for example, 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, and any value within the range of any two values ​​between them.

[0061] In this invention, the chemical formula of the lithium manganese iron phosphate matrix has 4≤q≤5, for example, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, and any value within the range of any two values ​​between them.

[0062] This invention rationally controls the values ​​of a, x, y, j, and q in the chemical formula of lithium manganese iron phosphate matrix, thereby effectively improving the electrochemical performance of lithium manganese iron phosphate cathode materials. Specifically, adjusting the value of a controls the number of lithium vacancies in the crystal lattice, thus affecting lithium-ion diffusion kinetics and structural stability. Adjusting the value of x controls the iron content, thus affecting the voltage plateau and electronic conductivity. Adjusting the value of y controls the manganese content, thus determining the high voltage plateau. Adjusting the value of j controls the dopant element content, optimizing the material structure through bulk doping or surface modification. Adjusting the value of q controls the phosphorus-oxygen ratio, maintaining the stability of the olivine structure and regulating the lattice energy. Preferably, the carbon coating layer accounts for 0.5% to 3% of the total mass of the lithium manganese iron phosphate cathode material, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, and any value between them. In this invention, the carbon content within the above range can balance the conductivity and specific surface area of ​​the cathode material, thereby improving the electrochemical performance, compaction density, and interfacial stability of the cathode material.

[0063] The lithium secondary battery of this invention also includes a battery casing, and the battery pack is disposed inside the battery casing. The battery casing can be any battery casing used in lithium batteries in the prior art. For example, the battery casing is mainly made of metal (steel, aluminum) and polymer (plastic, composite film), and the structure is divided into rigid casing (cylindrical / square) and flexible casing (soft pack). As long as it can encapsulate the battery cell, protect the internal structure and ensure safety, it can be used in this invention.

[0064] The lithium-ion secondary battery of this invention exhibits excellent high cycle stability, significantly improving the cycle life of lithium-ion secondary batteries using lithium manganese iron phosphate cathode materials. It has wide applications in electric vehicles, energy storage systems, and portable electronic devices. With technological advancements, these fields demand increasingly higher battery cycle life. This invention effectively improves the cycle life of lithium manganese iron phosphate batteries, meeting the performance requirements of these fields. Simultaneously, the development of electric vehicles places higher demands on battery high-temperature stability and cycle life under high-voltage platforms. The application of this invention can improve the range and safety of electric vehicles, promoting their development. Furthermore, energy storage systems, as a crucial component of new energy sources, also have high requirements for battery high-temperature stability and cycle life. The application of this invention can improve the energy storage efficiency and reliability of energy storage systems, further promoting the development of new energy sources. Therefore, this technical solution has broad market demand and promising application prospects.

[0065] In a second aspect, the present invention provides an electrical device comprising the lithium secondary battery described in the first aspect.

[0066] In some embodiments, the electrical device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems. To meet the high power and long cycle life requirements of the lithium manganese iron phosphate secondary battery for this electrical device, a battery pack or battery module can be used.

[0067] In other embodiments, the power device can be a mobile phone, tablet computer, laptop computer, etc. This power device is typically required to be thin and lightweight, and may also use a lithium manganese iron phosphate secondary battery as its power source.

[0068] The embodiments of the present invention described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0069] Example 1

[0070] Electrolyte provided: The electrolyte contains 1 wt% tris(trimethylsilane)phosphate (TMSP), 1 wt% vinylene carbonate (VC), 13 wt% electrolyte, and organic solvent. The electrolyte is LiPF6, and the balance is organic solvent. The organic solvent is ethylene carbonate and dimethyl carbonate in a 1:1 mass ratio. Therefore, the mass ratio of vinylene carbonate to tris(trimethylsilane)phosphate is 1:1, and the mass ratio of LiPF6 to tris(trimethylsilane)phosphate is 13:1.

[0071] Prepare the positive and negative electrodes: The positive electrode material is LiFe. 0.4 Mn 0.6 PO4 / C, with a C content of 1.5 wt%. The negative electrode material is graphite. The positive and negative electrode materials are coated onto aluminum foil and copper foil, respectively, with a coating surface density of 28 mg / cm³ for the positive electrode material. 2 The surface density of the negative electrode material coating is 13 mg / cm³. 2 They are made into sheet-like positive and negative electrodes.

[0072] Assemble lithium secondary batteries: such as Figure 5 As shown, a stacked battery is assembled from a positive electrode 1, a negative electrode 2, a separator 3, an electrolyte, and a battery casing 4. Specifically, a battery pack is arranged inside the battery casing. The battery pack includes alternating positive electrodes 1 and negative electrodes 2, with the outermost layer of the battery pack consisting entirely of positive electrodes 1. An electrolyte-containing separator 3 is placed between the positive electrodes 1 and negative electrodes 2, forming a stacked structure. The battery assembly is completed in a glove box filled with high-purity argon gas (H2O < 0.5 ppm, O2 < 0.5 ppm). Multiple sample batteries are assembled for subsequent testing. Both positive electrodes 1 and negative electrodes 2 are sheet-like electrodes, and the outermost layer of the battery consists entirely of positive sheet electrodes.

[0073] Example 2

[0074] The procedure was carried out in accordance with Example 1, except that the electrolyte contained 2 wt% TMSP and 1.5 wt% VC, as well as an organic solvent. The electrolyte was 16 wt% LiPF6, with the remainder being an organic solvent. The organic solvents included propylene carbonate and diethyl carbonate in a mass ratio of 2:1. This resulted in a calculated mass ratio of vinylene carbonate to tris(trimethylsilane)phosphate of 0.75:1 and a mass ratio of LiPF6 to tris(trimethylsilane)phosphate of 8:1.

[0075] Example 3

[0076] The procedure was carried out in accordance with Example 1, except that the mass content of tris(trimethylsilane)phosphate in the electrolyte was adjusted to 2 wt%, while the amount of vinylene carbonate remained unchanged. The calculated mass ratio of vinylene carbonate to tris(trimethylsilane)phosphate was 0.5:1, and the mass ratio of LiPF6 to tris(trimethylsilane)phosphate was 6.5:1.

[0077] Example 4

[0078] The procedure was carried out in accordance with Example 1, except that the mass content of vinylene carbonate in the electrolyte was adjusted to 1.5 wt%, the amount of tris(trimethylsilane) phosphate remained unchanged, and the mass ratio of vinylene carbonate to tris(trimethylsilane) phosphate was 1.5:1.

[0079] Example 5

[0080] The procedure was carried out in accordance with Example 1, except that the mass content of tris(trimethylsilane) phosphate in the electrolyte was adjusted to 0.5 wt%, while the amounts of vinylene carbonate and electrolyte remained unchanged. The calculated mass ratio of vinylene carbonate to tris(trimethylsilane) phosphate was 2:1, and the mass ratio of LiPF6 to tris(trimethylsilane) phosphate was 26:1.

[0081] Example 6

[0082] The procedure was carried out in accordance with Example 1, except that the electrolyte in the electrolyte solution was adjusted to 16 wt% LiPF6, so that the mass ratio of LiPF6 to tris(trimethylsilane)phosphate was 16:1.

[0083] Example 7

[0084] The procedure was carried out in accordance with Example 1, except that the organic solvents were adjusted to include propylene carbonate and diethyl carbonate in a mass ratio of 2:1.

[0085] Comparative Example 1

[0086] The same procedure was followed as in Example 1, except that the outermost layer of the battery pack was adjusted so that one side was the positive electrode and the other side was the negative electrode, instead of being entirely positive.

[0087] Comparative Example 2

[0088] The procedure was carried out in accordance with Example 1, except that tris(trimethylsilane)phosphate TMSP was replaced with tris(trimethylsilane)borate ester (TMSB), while the mass remained the same.

[0089] Comparative Example 3

[0090] The procedure was carried out in accordance with Example 1, except that the lithium manganese iron phosphate cathode material was replaced with another material: lithium iron phosphate.

[0091] Test case

[0092] The lithium secondary batteries obtained in the above embodiments and comparative examples were subjected to performance tests:

[0093] 1. First cycle performance test: Under the condition of 25℃ in the test constant temperature chamber, the test steps are as follows: S1, 1C constant current charging to 4.25V, then constant voltage charging to the cutoff current 0.05C, and stand for 30 minutes; S2, 1C constant current discharging to 2.5V, and stand for 30 minutes; S3, repeat steps S1 and S2, with a cycle number of 500.

[0094] 2. Second cycle performance test: Under the condition of 45℃ in the test constant temperature chamber, the test steps are as follows: S1, 1C constant current charging to 4.25V, then constant voltage charging to the cutoff current 0.05C, and stand for 30 minutes; S2, 1C constant current discharging to 2.5V, and stand for 30 minutes; S3, repeat steps S1 and S2, with a cycle number of 500.

[0095] The capacity retention rate after 500 cycles at 25℃ is calculated using the following formula: Discharge capacity after 500 cycles / Discharge capacity after the first cycle × 100%. The capacity retention rate after 500 cycles at 45℃ is calculated similarly. The cycle test results are shown in Table 1. Specific testing methods for battery capacity can be found in GB / T 31486-2015. The capacity retention rate curves of the lithium secondary battery in Example 2 under ambient temperature (25℃) and high temperature (45℃) conditions are shown below. Figure 1 , Figure 2 As shown in the figure. The capacity retention curves of the lithium secondary battery of Comparative Example 1 under ambient temperature (25℃) and high temperature (45℃) conditions are respectively shown in the figure. Figure 3 , Figure 4 As shown.

[0096] Table 1

[0097] Performance indicators Capacity retention after 500 cycles at 25°C Capacity retention after 500 cycles at 45℃ Example 1 98.25% 97.46% Example 2 97.45% 96.32% Example 3 95.64% 92.96% Example 4 95.58% 92.86% Example 5 95.19% 92.82% Example 6 94.89% 92.62% Example 7 94.75% 92.32% Comparative Example 1 94.59% 91.66% Comparative Example 2 93.23% 90.35% Comparative Example 3 91.25% 88.63%

[0098] The results above show that, compared with the comparative example, the embodiment of the present invention can effectively improve the cycle life of lithium secondary batteries using lithium manganese iron phosphate cathode material, while also helping to improve the cycle stability of the battery, and still has excellent battery cycle performance under high temperature and high pressure.

[0099] Furthermore, as can be seen from Examples 1 and 2-7, the preferred scheme of this invention is more conducive to significantly improving the cycle stability of the battery and significantly enhancing the cycle life of the lithium secondary battery with lithium manganese iron phosphate cathode material. Among them, the battery of Example 1, after 500 cycles at 25°C, maintained a capacity retention rate of 98.25%, demonstrating good cycle stability.

[0100] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium secondary battery, characterized in that, The device includes a battery pack and a battery casing. The battery pack is disposed within the battery casing. The battery pack includes alternating positive and negative electrodes, with the outermost layer of the battery pack consisting entirely of positive electrodes. A separator containing an electrolyte is disposed between the positive and negative electrodes. The positive electrode comprises lithium manganese iron phosphate, and the electrolyte comprises an electrolyte, an organic solvent, and additives. The additives contain tris(trimethylsilane) phosphate, with a mass content of 0.5-3 wt%. The organic solvent comprises cyclic carbonates and / or chain carbonates. The cyclic carbonates include at least one of ethylene carbonate, propylene carbonate, and butene carbonate, while the chain carbonates include at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. The tris(trimethylsilane) phosphate undergoes oxidative decomposition to form a thin and dense protective film. This protective film prevents direct contact between the organic solvent and the surface of the battery casing. The battery casing has a high potential.

2. The lithium secondary battery according to claim 1, characterized in that, The additive also contains vinylene carbonate.

3. The lithium secondary battery according to claim 2, characterized in that, The mass ratio of vinylene carbonate to tris(trimethylsilane) phosphate is (0.5-2):

1.

4. The lithium secondary battery according to claim 2, characterized in that, The mass content of vinylene carbonate in the electrolyte is 0.25-6 wt%.

5. The lithium secondary battery according to claim 1, characterized in that, The electrolyte includes lithium hexafluorophosphate, and the mass ratio of lithium hexafluorophosphate to tris(trimethylsilane) phosphate is (5-35):

1.

6. The lithium secondary battery according to claim 5, characterized in that, The mass of lithium hexafluorophosphate is 10wt%-17wt% of the total mass of the electrolyte.

7. The lithium secondary battery according to claim 1, characterized in that, When the organic solvent includes cyclic carbonates and chain carbonates, the mass ratio of cyclic carbonates to chain carbonates is (0.5-5):

1.

8. The lithium secondary battery according to claim 1, characterized in that, Lithium secondary batteries also include at least one of the following structures: Structure 1: Both the positive and negative electrodes are sheet-like electrodes, and / or the battery pack is a stacked structure formed by the positive and negative electrodes and the separator; Structure 2: The negative electrode includes a negative electrode active material, which includes at least one of carbon, silicon-carbon, and silicon negative electrode.

9. The lithium secondary battery according to claim 1, characterized in that, The lithium manganese iron phosphate cathode material comprises a lithium manganese iron phosphate matrix and a carbon coating layer covering the surface of the lithium manganese iron phosphate matrix; the chemical formula of the lithium manganese iron phosphate matrix is ​​Li. a Fe x Mn y M j PO q Wherein, M includes at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, and La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, and 4≤q≤5; the mass of the carbon coating layer accounts for 0.5%-3% of the total mass of the lithium manganese iron phosphate cathode material.

10. An electrical appliance, characterized in that, It includes lithium secondary batteries as described in any one of claims 1-9.

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